1. Field of the Invention
The invention generally relates to a vehicle control system and a vehicle control method, and more particularly to a vehicle control system and a vehicle control method which provide desired vehicle body force and yaw moment by controlling only the steering angles of the front and rear wheels in a normal region in which an unused grip range is large, thereby to suppress unnecessary braking, and which is able to efficiently perform coordinated control of braking/driving control and steering control in regions, including a limit region in which the unused grip range is small, other than the normal region.
2. Description of Related Art
As a conventional technique for controlling the steering angles and braking/driving forces of all of the four wheels to be controlled, independently of one another, coordinated control for controlling the steering angle and braking/driving force of each wheel in a coordinated fashion as disclosed in Japanese Laid-open Patent Publication No. 2004-249971 is known. This technique is to provide a combination of the tire forces of the four wheels, which is selected from some combinations of the tire forces of the four wheels that achieve the target resultant force applied to the vehicle body and target yaw moment, such that the selected combination minimizes the μ utilization ratio of each wheel (i.e., the ratio of the tire force of each wheel to the maximum value thereof), namely, maximizes an unused grip range of a tire of each wheel. Here, it is to be noted that the μ utilization ratio and the unused grip range have a relationship that “unused grip range=1−μ utilization ratio”. A known integrated control logic for controlling the steering angles and braking/driving forces of the four wheels in an integrated fashion provides an algorithm that minimizes the μ utilization ratio of the wheel having the largest μ utilization ratio, out of the four wheels, thereby to achieve the theoretical limits of the vehicle body force and yaw moment in a region in which the tire forces of all of the four wheels are used. Thus, the known technique makes it possible to utilize the tire forces with high efficiency, and greatly contributes to an improvement of the dynamic performance of the vehicle while it is running in a limit region in which the unused tire grip range is important or critically small.
If the algorithm of the known technique is used, the dynamic performance of the vehicle can be effectively controlled in the limit region in which the unused tire grip range is important or critically small, but a steering actuator and a braking/driving actuator comprising a brake actuator and a driving actuator are also operated in a normal region in which the unused grip range is large, as well as in the limit region. The operation of the brake actuator causes the speed of the vehicle to be reduced, and the reduction in the vehicle speed may make the driver uncomfortable in the normal region. If the driving actuator is operated so as to compensate for the reduction of the vehicle speed, the fuel economy or efficiency may undesirably deteriorate.